Consolidated Statements of Changes in Shareholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Cumulative Effect Period Of Adoption Adjustment [Member]	Common Stock [Member]	Capital Surplus [Member]	Retained Earnings [Member]	Retained Earnings [Member] Cumulative Effect Period Of Adoption Adjustment [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance at Dec. 31, 2018	$ 1,591,453		$ 13,717	$ 309,545	$ 1,323,870		$ (55,679)
Balance (in shares) at Dec. 31, 2018			65,834,395
Net income per consolidated statements of income	33,339				33,339
Other comprehensive income (loss), net of tax	14,764						14,764
Common stock dividends paid ($0.23 per share)	(15,033)				(15,033)
Shares withheld to pay taxes, long-term incentive plan	(1,604)		$ 25	(1,629)
Shares withheld to pay taxes, long-term incentive plan (in shares)			123,821
Repurchase and retirement of common stock	(36,893)		$ (243)	(36,650)
Repurchase and retirement of common stock (in shares)			(1,168,273)
Compensation expense, long-term incentive plan	1,002			1,002
Balance at Mar. 31, 2019	1,587,028		$ 13,499	272,268	1,342,176		(40,915)
Balance (in shares) at Mar. 31, 2019			64,789,943
Balance at Dec. 31, 2018	1,591,453		$ 13,717	309,545	1,323,870		(55,679)
Balance (in shares) at Dec. 31, 2018			65,834,395
Net income per consolidated statements of income	116,514
Other comprehensive income (loss), net of tax	34,821
Compensation expense, long-term incentive plan	3,553
Balance at Sep. 30, 2019	1,645,362		$ 13,390	257,370	1,395,460		(20,858)
Balance (in shares) at Sep. 30, 2019			64,262,779
Balance at Mar. 31, 2019	1,587,028		$ 13,499	272,268	1,342,176		(40,915)
Balance (in shares) at Mar. 31, 2019			64,789,943
Net income per consolidated statements of income	42,140				42,140
Other comprehensive income (loss), net of tax	16,099						16,099
Common stock dividends paid ($0.23 per share)	(14,987)				(14,987)
Shares withheld to pay taxes, long-term incentive plan	(1)		$ 2	(3)
Shares withheld to pay taxes, long-term incentive plan (in shares)			7,087
Repurchase and retirement of common stock	(13,008)		$ (83)	(12,925)
Repurchase and retirement of common stock (in shares)			(398,184)
Compensation expense, long-term incentive plan	1,279			1,279
Balance at Jun. 30, 2019	1,618,550		$ 13,418	260,619	1,369,329		(24,816)
Balance (in shares) at Jun. 30, 2019			64,398,846
Net income per consolidated statements of income	41,035				41,035
Other comprehensive income (loss), net of tax	3,958						3,958
Common stock dividends paid ($0.23 per share)	(14,904)				(14,904)
Shares withheld to pay taxes, long-term incentive plan	(34)		$ 1	(35)
Shares withheld to pay taxes, long-term incentive plan (in shares)			2,502
Repurchase and retirement of common stock	(4,515)		$ (29)	(4,486)
Repurchase and retirement of common stock (in shares)			(138,569)
Compensation expense, long-term incentive plan	1,272			1,272
Balance at Sep. 30, 2019	1,645,362		$ 13,390	257,370	1,395,460		(20,858)
Balance (in shares) at Sep. 30, 2019			64,262,779
Balance at Dec. 31, 2019	$ 1,660,702	$ (19,949)	$ 13,376	256,400	1,414,526	$ (19,949)	(23,600)
Balance (in shares) at Dec. 31, 2019	64,200,111		64,200,111
Net income per consolidated statements of income	$ 22,218				22,218
Other comprehensive income (loss), net of tax	31,298						31,298
Common stock dividends paid ($0.23 per share)	(14,706)				(14,706)
Shares withheld to pay taxes, long-term incentive plan	(1,020)		$ 17	(1,037)
Shares withheld to pay taxes, long-term incentive plan (in shares)			83,759
Repurchase and retirement of common stock	(27,538)		$ (184)	(27,354)
Repurchase and retirement of common stock (in shares)			(886,958)
Compensation expense, long-term incentive plan	1,394			1,394
Balance at Mar. 31, 2020	1,652,399		$ 13,209	229,403	1,402,089		7,698
Balance (in shares) at Mar. 31, 2020			63,396,912
Balance at Dec. 31, 2019	$ 1,660,702	$ (19,949)	$ 13,376	256,400	1,414,526	$ (19,949)	(23,600)
Balance (in shares) at Dec. 31, 2019	64,200,111		64,200,111
Net income per consolidated statements of income	$ 108,808
Other comprehensive income (loss), net of tax	29,284
Compensation expense, long-term incentive plan	3,907
Balance at Sep. 30, 2020	$ 1,710,041		$ 13,215	231,836	1,459,306		5,684
Balance (in shares) at Sep. 30, 2020	63,423,820		63,423,820
Balance at Mar. 31, 2020	$ 1,652,399		$ 13,209	229,403	1,402,089		7,698
Balance (in shares) at Mar. 31, 2020			63,396,912
Net income per consolidated statements of income	32,150				32,150
Other comprehensive income (loss), net of tax	2,867						2,867
Common stock dividends paid ($0.23 per share)	(14,687)				(14,687)
Shares withheld to pay taxes, long-term incentive plan	(59)		$ 5	(64)
Shares withheld to pay taxes, long-term incentive plan (in shares)			25,527
Compensation expense, long-term incentive plan	1,274			1,274
Balance at Jun. 30, 2020	1,673,944		$ 13,214	230,613	1,419,552		10,565
Balance (in shares) at Jun. 30, 2020			63,422,439
Net income per consolidated statements of income	54,440				54,440
Other comprehensive income (loss), net of tax	(4,881)						(4,881)
Common stock dividends paid ($0.23 per share)	(14,686)				(14,686)
Shares withheld to pay taxes, long-term incentive plan	(15)		$ 1	(16)
Shares withheld to pay taxes, long-term incentive plan (in shares)			1,381
Compensation expense, long-term incentive plan	1,239			1,239
Balance at Sep. 30, 2020	$ 1,710,041		$ 13,215	$ 231,836	$ 1,459,306		$ 5,684
Balance (in shares) at Sep. 30, 2020	63,423,820		63,423,820